Phase-change memory (also known as PCM, PRAM, PCRAM, Chalcogenide RAM and C-RAM) is a type of non-volatile memory device that employs a reversible phase-change material to store information.
Phase-change memory uses a medium such as chalcogenide, the physical state of which can be reversibly changed between crystalline and amorphous through the application of heat. The physical states have different electrical resistance properties that can be easily measured, making chalcogenide useful for data storage.
In the amorphous phase, the material is highly disordered, that is there is an absence of regular order to the crystalline lattice. In this phase, the material demonstrates high resistivity and high reflectivity. In contrast, in the crystalline phase, the material has a regular crystalline structure and exhibits low reflectivity and low resistivity.
Phase-change memory uses electrical current to trigger the structural change. An electrical charge just a few nanoseconds in duration melts the chalcogenide in a given location. When the charge ends, the location's temperature drops so quickly that the disorganized atoms freeze in place before they can rearrange themselves back into their regular, crystalline order.
Going in the other direction, the process applies a longer, less-intense current that warms the amorphous patch without melting it. This energizes the atoms just enough that they rearrange themselves into a crystalline lattice, which is characterized by lower energy or electrical resistance.
To read recorded information, a probe measures the electrical resistance of the location. The amorphous state's high resistance is read as a binary 0, while the lower-resistance, crystalline state is a 1.
FIG. 6 illustrates a cross-section of a phase-change memory cell 600 formed over a substrate 610. The substrate 610 includes a conductive line 620 coupled to a selection device 630. The selection device 630 may be, for example, a diode, a transistor, or other similar device. The selection device 630 is electrically coupled to a lower electrode 640 formed in a pore 650. The pore 650 is defined as an aperture in an insulating layer 660. Sidewall spacers 670 are formed in the pore 650. A phase-change material 680 is formed in the pore 650 and over the insulator 660. An upper electrode or conductive line 690 is formed over the phase-change material 680. The phase-change material 680 may be set to a desired resistance by varying the magnitude of the applied current.
Semiconductor chips have been the subject of attacks to read data or manipulate circuit operation. As an example, one type of attack technique involves looking through a substrate of the semiconductor chip from the rear using an infrared laser having a wavelength at which the substrate is transparent. The photocurrents created enable probing of the semiconductor chip operation and identification of logic states of individual transistors.
There have been numerous techniques used to prevent such attacks. For example, semiconductor chips have been formed with multiple layers to hide sensitive data lines.
Protective layers have also been used to prevent analysis of real time data processing. A top layer may have an active grid carrying a protection signal. Interruptions of the protection signal cause the semiconductor chip to erase its memories and cease operation.
Protective circuits have also been used to protect the semiconductor chip by preventing unauthorized retrieval of the secure information.
Conductive bridging memory devices have also been used in combination with a photodiode to detect an unauthorized manipulation or access. The photodiode is used as the sensor, converting optical energy to electrical energy. The conductive bridging memory device is electrically switched using the electrical energy. Thus, both the photodiode as well as a memory device are needed to detect the attack.
Another type of attack involves placing probes or needles on the semiconductor chip and listening to information, or forcing specific signals on the semiconductor chip in order to generate a behavior supporting the attack.
One protection mechanism against a probe attack involves a passive or active shield placed on top of security critical portions of the semiconductor chip, so that an attacker can not directly read the chip's signals. Passive shields are typically effective in preventing viewing of the chip and making attacks more time-consuming. Passive shields may be removed, however, without affecting the operation of the device. Active shields may look similar to passive shields. However, a breach in an active shield is typically detected and normally results in disabling the chip.
Another protection mechanism involves using a specific encryption or masking of the signals, rendering the signals useless to an attacker.